Optical storage of digital data is a relatively new technology, concerned with the storage and retrieval of digital information utilizing optical techniques and using a special related (ODD, "optical digital data") medium, such as an ODD disk. By analogy such data is conventionally stored on magnetic media like tapes or disks commonly used with high speed digital computers today.
Here described are some novel approaches to making a sensitive optical recording medium for digital data, resisting oxidation or like environmental degradation, wherein sensitivity is improved, extended life is feasible and fabrication parameters are simplified over what is now conventional.
Various types of protective overcoatings for such media have been suggested by workers, especially relative to "tuned media" (e.g., media using a "dark mirror" effect; for instance see U.S. Pat. No. 4,222,071 to Bell, et al; also see "Review of Optical Storage Media" by Zech, SPIE Vol. 177, Optical Information Storage, 1979, page 56, et sequ.; also see "Optical Recording Media Review" by Bartolini, page 2, et sequ. of 1977 SPIE Vol. 123, "Optical Storage Materials and Methods"; and see "Melting Holes in Metal Films for Real-Time High Density Data Storage" by Cochran and Ferrier, SPIE Proceedings, August 1977, pages 17-31; and other citations below).
Extended Archival life:
Optical data storage technology is attractive because it promises increased storage capacity. An optical data disk as here contemplated will be assumed to store information thereon for an extended archival life; the goal is 5-10 years or more under typical, and extreme, service conditions for data processing (DP) apparatus. Such extended life is a goal as yet unattained in the art, though workers have long striven towards it. The present invention points toward improved ODD media better adapted for such archival life; media which are especially adapted for "optical mass memory" and like applications, with emphasis on improved overcoat and/or spacer means.
Thus, as a feature hereof, we contemplate the use of novel spacer and/or overcoat structure and materials for records which preferably exhibit extended archival life, i.e., records which are made extremely resistant to oxidation or like environmental degradation during typical DP storage and use (thus, with little or no "loss" of recorded information occurring over extended storage life, with reflectivity remaining stable enough to "read")--something no practical storage medium or associated system can yet provide; especially where "good" sensitivity is also required. The invention teaches means toward this end.
Novel "spacer layer":
Further, according to a salient aspect hereof, the spacer layer (e.g., in such a "dark mirror" arrangement) may preferably comprise a "soft pad" which is vapor-deposited on a reflector layer, and upon which the absorber (recording) layer may in turn be deposited. This spacer layer may comprise a so-deposited fluoropolymer which is highly transparent to the contemplated read-write wavelengths and which also provides good thermal and mechanical insulation, isolating the absorber layer from the reflector layer, (note the reflector is typically a highly conductive metal which could otherwise function as a heat sink, draining recording energy away from the absorber layer and reducing its effectiveness).
Thus, as further described below, for one example we prefer a vacuum-evaporated fluoropolymer, like polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene copolymer (FEP).
It has been somewhat conventional to specify a hard silicate coating (silicon oxide or silicon dioxide--cf "fused silica") for such a spacer (e.g., see U.S. Pat. Nos. 4,195,312 or 4,195,313 or 4,216,501 to Bell, et al). However, such material cannot satisfy all the thermal and mechanical objectives here set out. For one thing, such silicate coatings conduct heat too readily. For example, a "low-energy" recording pulse (moderate-to-low power for about 40 n.sec) has difficulty recording when a single quarter-wave optical thickness of SiO.sub.2 is used (atop Al reflector)--this very thin spacer evidently allows too much recording heat to escape and be wasted. To prevent this, thicker spacer layers are possible, of course; however, thicker SiO.sub.2 layers can complicate and degrade production processes, especially because they typically will constrict the absorber and drop sensitivity and because they can narrow the bandwidth where low reflectivity obtains (cf. "Design and Production of Tellurium Optical Data Disk" by J. Rancourt, SPIE Proceedings; Advances in Laser Scan Technology, page 57 Vol. 299, 1981).
Moreover, if several well-separated read/write R/W) wavelengths are to be used, spacer production becomes even more complex since more (thickness) control is required to accurately position the necessary wavelength "minima". (Note: One wants to efficiently couple light energy to the absorbing layer of a "tri-layer"; hence, to "write" by increasing the reflectivity of a bit site, one wants an unwritten background of relatively low reflectivity, at contemplated R/W wavelengths). Conversely, a simple "soft pad" (e.g. fluoropolymer) spacer using this teaching appears to affect sensitivity much less; also it affords a broad reflectance minimum (.lambda.) by virtue of being only a single quarter-wave and can therefore function at many R/W wavelengths simultaneously.
One aspect of this disclosure is to teach the preparation and use of such a "soft pad" spacer material, including associated deposition methods, and especially such apt for such OD disks--and even more especially such which are typically convenient for low-energy recording with present laser equipment (e.g., writing with a He-Ne laser in a 5-20 mW/40 n.sec pulse--cf 25 MHz rate).
Overcoat; generally:
The so-recorded spots ("bits") are contemplated as being about one micrometer in diameter. But surface "dirt" (e.g., oil, fingerprints) or particulate contaminants, such as air-borne dust, are this large, or larger, and thus can obstruct a recorded "bit". For instance, common smoke particles can be about six microns (6 um, or about 240 microinches) in diameter. Consequently, such contaminant particles will commonly "mask", and so obliterate, recorded "bits" (data) if one or several of them sits just above on the overcoat.
So, it has become conventional to specify a thick overcoating layer for defocusing such contaminant particles and all smudges, spots or smears--e.g., here, by providing a transparent overcoating on the order of 100 to 180 micrometers thick. Thus, any dust particles that do settle on the surface of such a protective layer, (and are not wiped-away) will be "defocused"; i.e., thrown out of the focal range of the objective used to detect recorded data and the rest of the optical train--optically they "disappear". As a second purpose, such an overcoat should provide mechanical protection for the recording layer and prevent damage from handling, etc. (e.g. during fabrication, testing or service).
Now, in some cases, workers have suggested relatively "hard" materials as a protective transparent overcoat, while in others they have proposed "softer" materials. For instance, some have suggested an elastomer outer-coat (cf a silicone rubber like "Silastic RTV" by GE--see U.S. Pat. No. 4,101,907, to Bell, et al where an "ablatable" absorber, such as certain organic dyestuffs, was overcoated with a "barrier layer" of SiO.sub.2 , or of derivatives of sucrose or resin acids; and this super-coated with such a silicone resin). But known overcoatings of soft, resilient (rubbery) materials have characteristically exhibited a "tacky" exposed surface which readily attracts and retains dust; and in certain instances, such "elastomeric" coatings still seem to "constrict" the underlying absorber. Also, elastomers may require a curing temperature that is too high; or, if they cure at room temperature it may take far too long; yet, when heated for "quick curing" they present a serious risk of overheating the tri-layer (--a silicone elastomer like RTV presents all these shortcomings, along with cure-stress, and excessive moisture-uptake in service).
On the other hand, other workers have considered a "hard" outer "sealing" overcoat applied directly over the absorbing layer (e.g., see "Optical Disk Systems Emerge" by Bartolini, et al IEEE Spectrum, August 1978, where, in a "tri-layer" structure, SiO.sub.2 is specified above and below a titanium absorber); yet they have been forced to concede that, such a hard overcoat (perhaps because it unyieldingly confines and constricts the absorber) appears to degrade recording sensitivity, to the point where it renders an otherwise acceptable recording medium essentially "unrecordable". Also, hard outer coatings like SiO.sub.2 are too absorptive (e.g., of water vapor) to be long-lived.
"Hard/soft" overcoat:
Thus, another aspect of this disclosure is to provide an overcoating which avoids most or all of the foregoing shortcomings, doing so by providing a two-part overcoating made up of a "soft pad" inner layer and a "hard" outer sealing layer--i.e. with a "Hard/Soft" overcoat. The soft inner pad is intended to be yielding and quite compressible (as a "mushy cushion") allowing the subjacent absorber to distort and/or move during write-heating, while also providing good thermal insulation (very low thermal conductivity; relatively low specific heat). In short, this "soft pad" seems to better isolate the absorber, mechanically and thermally; while the "hard" outer coat gives optimal mechanical protection (e.g. a seal against vapor entry). Of course, such layers should also bond well, be highly transparent to the contemplated read/write wavelengths and preferably be convenient and inexpensive to apply.
As mentioned, the mechanical properties of certain such "soft pads" (e.g. of FEP or PTFE, see below) appear to better accommodate motion or deformation of the underlying absorber during "write-heating" (e.g. as a "top pad"; also as a "bottom pad" if the soft material is used as a "spacer" too). Such "soft pads"--evidently because they so decouple the absorber, mechanically and thermally, from its surrounding environment--are found able to markedly increase "sensitivity" (e.g. well over what can be expected using only a "hard" overcoating like fused silica--i.e. the latter will require more energy to "write" a given bit or "hole"). A "soft pad" is so effective as such isolation that even where only used as a subjacent "spacer" (e.g. with SiO.sub.2 directly over absorber) it has been seen to enhance sensitivity (e.g., vs. replacing it with an SiO.sub.2 spacer).
And, as mentioned below, such a "soft pad" coating is preferably applied as a vacuum-evaporated film with essentially the same facilities as those used to deposit the absorber layer (e.g., during a related, succeeding deposition step, and with common equipment). The consequent convenience and reduced cost, time, etc. will be evident. According to this feature, we have discovered preferred materials satisfying most or all of these rather stringent "soft pad" requirements (including vacuum evaporation) such as the mentioned fluoropolymers (FEP and PTFE). And, advantageously, it is possible to use the same "soft pad" material for both sides of an absorber (i.e., as spacer and overcoat). Alternatively one may choose from a class of plasma polymerized polymers such as a polyethylene or "Parylene" (TM, as para-xylene by Union Carbide, cf U.S. Pat. No. 3,342,754). Preferably, such evaporo-deposition of a "soft pad" layer is applied at the same time, and with the same equipment as that for depositing the absorber layer (and/or the spacer layer).
The thickness of this "soft pad" overcoat is preferably such as to so decouple the absorber layer (thermally and mechanically) from any supercoating (especially a "hard" layer applied over the "soft pad") --and also to bond favorably with the underlying absorber (e.g. so that sensitivity is not badly compromised and so the absorber is suitably "decoupled" from a hard "outer" overcoating, while also preventing the hard overcoating, and/or any stress therefrom, from constraining the absorber and so interfering with pit-formation therein--yet bonded well enough to the "hard" coat to prevent "delamination", moisture intrusion, etc. in service, these easily upsetting the needed optical properties--cf. a mere 100 .ANG. shift can destroy the required "tuning").
It is important to protect the absorber from any such deleterious effects; for instance, especially where one uses absorbers which deform and/or are displaced in the course of recording and creating a "bit-spot". It will be apparent to workers that a hard overcoating (e.g. SiO or SiO.sub.2 as known) applied directly on the absorber layer can be expected to constrict it, and restrain such deformation or translation during "bit-writing"--thus interfering with bit formation and degrading sensitivity and recording efficiency, so that more write-energy is needed. Also, most silicon oxides absorb too much moisture. We have experienced these problems using SiO.sub.2 (evaporodeposited on a "cool" substrate)--much less so with materials like FEP or PTFE (cf these can be deposited as relatively "non-porous" films under like circumstances).
Thus, workers will see how important and useful a proper "soft pad" of the type described can be, especially where one wants to enhance the recording efficiency of an adjacent OD absorber layer.
Thus, another form of this feature is to so provide a "soft pad" coating over an absorber layer and, where possible to do so using common deposition techniques. A related feature is to provide a like "soft pad" spacer layer beneath the absorber in some instances--whereby one may thermally and mechanically isolate the absorber from interference generated from above and/or below.
A related-feature is to superpose a "hard" protective overcoating outward of this "soft pad", overcoat e.g. to serve as a good vapor barrier, and as a mechanical "cover" and an anti-static surface, as well as to complete the necessary optical thickness for "defocusing" surface contaminants--i.e., a "Hard/Soft" overcoat.
A further improvement on the above is to so apply a (fluoropolymer) "soft pad" which is sufficiently soft and yielding as to mechanically decouple the adjacent absorber layer, freeing it to "move" as written, while also isolating it thermally (i.e. to so function, either as a subjacent "spacer" or as an overlying "soft overcoat" or as both).
A further improvement is to so provide such a "soft pad" spacer using an organic layer which is made strongly adherent to an underlying reflector layer while also being relatively differently adherent to a superposed absorber layer. And a related feature is to provide such a "soft pad" overcoat which bonds to a superposed hard overcoat relatively firmly but bonds differently to a subjacent absorber.
Novel "Hard" supercoat:
As mentioned above, another salient feature hereof is that the above-characterized "soft pad" overcoat is, in turn, preferably super-coated with a compatible "hard" outer protective layer.
And, as a more specific feature, a family of novel "radiation-cured polymers" is here taught for such a "hard", outer coating for an archival OD (optical data) disk; also, a preferred associated novel method is taught for coating such disks with such material.
A novel pre-polymer formulation is described below (e.g. see Mix H-1); it is intended to provide such a "hard" protective overcoating for such OD disks (extended archival life, etc.) and especially as a super-coat over such a "soft pad" overcoat. More particularly, it is intended to provide a "clear" coating (transparent at the contemplated R/W wavelengths), of a thickness to help "defocus" surface dust, etc. (e.g., up to 6-8 mils here) and to provide an environmental barrier against mechanical interference or vapor intrusion (especially water, aqueous aerosols, sulfates or NaCL or other chlorides). It is intended to so function rather like known overcoatings (of a "glass" for instance), and to provide good mechanical protection, (e.g. allowing one to lightly squeeze the disk, though it need not resist a positive cutting action, such as scraping with fingernail--note: without some such a hard super-coating, the soft FEP layer can be wiped-off readily).
Known "hard" overcoatings:
Workers in this art have considered various materials for similar protective coatings. For instance, it has become common to suggest a "glassy" form of overcoat, such as with "fused silica" (SiO.sub.2, or SiO) but for present purposes (OD disks, etc.) these seem to be disqualified. For example, they are typically highly porous and can take-up too much moisture; thus they are too prone to swell and crack (especially under the mentioned extreme temperature/humidity cycling tests)--also such moisture contaminants badly degrade optical characteristics. Also, they are not optimal for the desired vacuum-evaporation deposition (e.g. impractical to so deposit several mils or more).
Besides such inorganic overcoatings, workers have considered certain organic materials for providing protective overcoats in similar situations. For instance, as mentioned, some workers have considered using a silicone rubber or like elastomeric polymer for this--e.g., some silastics which may be conveniently curable at room temperature, typically liberate harmful contaminants like acetic acid during cure, (or see "plastic sheet" of U.S. Pat. No. 4,334,233).
In a similar vein, we have considered using a known fluoropolymer, but in the thicknesses contemplated (6 to 8 mils) typical fluoropolymer deposition methods are not favored--e.g. typically require dissipating too much solvent (see problems below with solvent dissipation and associated shrinkage, etc.). More seriously, this could involve a cure-heating which is entirely too intense (at about 390.degree. C.), whereas the subject OD disks and associated coatings are not intended to survive more than aoout 66.degree. C. (e.g. otherwise their coatings, such as the organic soft FEP overcoat and the absorber layer, would be destroyed, and/or constituents could migrate, etc.). Moreover such polymers are apt to exhibit a "tacky", dust-retaining, surface and are not believed optimally transparent at the subject read/write wavelengths (cf. 600-900 N.meters).
Also considered for such a hard protective overcoat were various "solvent-based" (solvent-applied) polymers. However, drying (curing) these involves dissipating relatively large proportions of solvent, with a great deal of problematical shrinkage likely. This has seemed to disqualify these materials, especially for coatings as thick as those contemplated (also, bubbles, etc. would probably form in such a thick coating of these materials).
Also contemplated were various "two-component curing" polymers such as "RTV-6" (by GE). However, these are somewhat difficult to apply, typically having a relatively high viscosity (possibly requiring problematical heating to soften enough for quick, smooth application); they also typically present "out-gas" problems; further, many cure relatively slowly and at a relatively high temperature (e.g., 15 minutes at about 66.degree. C.--and, even then, the cured material often exhibits a tacky surface and is too apt to scratch, peel-off, etc.). Moreover,such materials typically have too brief a "pot-life" (on the order of one day)--yet another application shortcoming.
Another family of protective materials considered was the PVC type (polyvinyl chloride) but these seemed impractical because of the solvent-application involved; also, and more important, they will typically crystallize over time, giving rise to an unacceptable optical "dichroism". This "dichroism" will interfere with the desired read/write beam transmission (the read/write laser beams are already polarized, etc. and a crystallized overcoat would obviously give optical problems, as workers will recognize). The subject preferred radiation-cured acrylic polymers do not seem to present the foregoing problems, e.g., they do not so crystallize and present no "dichroism" problem.
Preferred materials for "HARD overcoat":
Accordingly, the foregoing families of chemical coatings are disfavored. An attempt was made at using a "radiation-cured" acrylic type polymer (acrylic monomer, or pre-polymer mix with various additives, similar to the "Mixture H-1" discussed below). It was found, somewhat surprisingly, that when properly applied (e.g. see "spiral" technique, below; with appropriate "setting surfactant" and appropriate "solvent-leveling", etc.) such an overcoat could satisfy (most, if not all of) the mentioned requirements, whereas other materials seem less likely able to do so. Thus, it is another object of this disclosure to teach the use of such radiation-cured polymers (especially acrylics) as a "hard" proteqtive overcoat for such optical data disks, as well as teaching related methods of preparing and applying them.
As detailed below, a preferred family of hard coat materials--"radiation-cured polymers"--is made up of a number of "acrylated monomers" (or "pre-polymers", i.e. a ligomer or resin that will undergo further polymerization --especially where the principal constituent is a suitable acrylate or acrylamide). A preferred version (Mix H-1) includes an appropriate acrylated epoxide and acrylated acrylate, together with an acrylate cross-linker and associated acrylate diluent plus photo-initiator, and preferably including a prescribed "setting surfactant". Also, a minor portion (e.g., 10%) of the Mix may comprise one or more additives (preferably organics which will participate in the UV polymerization, e.g. styrene or like vinyl-ethers do this).
Such acrylics are evidently eminently suitable for several reasons: they do not include (any significant portion of) problematic components like "shrink-prone solvents" and they require no problematic cure conditions (such as extreme heat). They seem to be especially apt for providing a final "Hard" polymeric overcoat which has the required characteristics.
And, such "acrylic radiation-cured polymers" will be recognized as satisfying essentially all the other cited requisites of the desired "Hard overcoat"; i.e. they don't readily crystallize, they have no massive solvent content or associated shrinkage problems, they are cured quickly and conveniently and without excessive heating; and they are relatively easy to apply, (e.g., as a low-viscosity monomer solution). They appear quite superior in resisting degradation and attack by common environmental components; they are not "tacky" or dust-retentive, and, unlike the "two-component-cured" polymers, they are compatible with a wide number and variety of additives (e.g. their curing is not affected thereby, as seen in the Examples below).
Workers will recognize that the required cure-radiation may be something as inexpensive, quick and convenient as a few seconds exposure to a UV source (of appropriate .lambda., intensity, etc.) and involve as little as 5% shrinkage. Or, where cost is not a major concern, one may instead cure with electron-beam or gamma radiation. Alternatively, a peroxide (catalyst) curing may be feasible in some cases. And, whatever the primary curing mode, it will be understood that light supplemental heat may, in certain cases, be so applied to hasten complete curing.
Application as "spirals":
According to a related feature, such acrylic overcoat polymers are preferably applied in a spiral configuration on a host substrate-disk, being evenly distributed thereon (e.g., with appropriate disk rotation and inclusion of a certain particular fluoropolymer "setting-agent"), and allowed (or in some cases induced) to settle and flow-out evenly. This is seen to spread the mix across this surface with exceptional smoothness and uniform thickness --e.g. giving a thickness variation as little as .+-.0.7 to 7.0 micrometers in a 7 mil coating across a disk-band several inches wide (e.g., the outer-half of a 14" disk). This is quite striking. Workers in the art, whether involved with such acrylic coatings or other coatings, will recognize that the simplicity and novel advantages of such a coating technique are quite remarkable.
The related discovery of a particular "setting-agent" (mentioned above) is, surprisingly, seen to not only provide the usual surfactant properties (such as enhancing wetting, leveling, etc.) to a remarkable degree, but also to "set" the mix--i.e. to cause the spiral rows, once applied to the host disk, to "set up" and remain in place until self-leveling (or being contacted with an appropriate leveling-solvent). Such a "setting action" will be seen to enhance the simplicity of the coating technique; for instance, it allows the host disk surface to be slowly rotated in the course of applying these spiral rows without the material deforming or moving asymmetrically under associated centrifugal forces.
One variant of this novel application technique includes a "solvent leveling" step wherein the spiral rows of the mixture so-applied on the disk, may be induced, upon contact by a proper solvent vapor, to "wet" (with the disk surface) and spread themselves across the disk surface with remarkable uniformity and speed. Particular solvents are suggested for this, especially apt for the preferred coating materials and substrate surface.
Thus, it is an object hereof to provide the foregoing, and other related, features and advantages. A more particular object is to do so, teaching the use of "soft pad" materials adjacent an "optical recording layer". Another object is to teach such for improved recording sensitivity, adequate for low-power lasers; as well as for extended service life. A further object is to teach preparation of such "soft pad" layers using fluoropolymer materials, especially as deposited by vacuum evaporation. Another object is to provide such "hard" overcoatings and associated preferred materials and application techniques.